The current generation of hyperspectral imaging (HSI) systems for use in the infrared wavelength bands employ a focal plane array (FPA) with a broad spectral response, integrated in a diffraction grating based on a Fourier transform spectrometer. An HSI system would be more widely deployable and available for applications if the system is miniaturized into a small package in an integrated microsystem without sacrificing the performance that existing systems provide. Recently, an approach for miniaturizing a hyperspectral imaging system on a chip has been demonstrated by integration of the broadband detection capability of a HgCdTe infrared detector with an electronically tunable filter, all on a single chip. Narrowband wavelength tunability is achieved by a Micro-Electro-Mechanical-System (MEMS) based tunable Fabry Perot (F-P) filter fabricated directly on a photodetector. A readout integrated circuit (ROIC) serves to both integrate the detected signal as well as electronically tune the filter across the wavelength band. The F-P filters use dielectric mirrors in the form of distributed Bragg reflectors, which are formed from alternating layers of high and low refractive index, low loss materials such as Ge and SiOx.
A key shortcoming of this approach is that the spectral tuning range of a F-P filter is generally limited to one octave in bandwidth range. In practice, the range is often lower due to the limitations in spectral width of the reflectors used to form the F-P resonant cavity. It is therefore not possible to use a single F-P cavity filter to tune over the wavelength range of, for example, 1.0-2.5 μm or the shortwave infrared (SWIR) band, a spectral band of considerable practical interest.
Therefore, there is a need for a spectrally tunable infrared detector or sensor that overcomes the problems noted above and others previously experienced for detecting a wide range (e.g., more than one octave) of infrared wavelengths.